KVM: arm64: Implement PSCI SYSTEM_SUSPEND

ARM DEN0022D.b 5.19 "SYSTEM_SUSPEND" describes a PSCI call that allows
software to request that a system be placed in the deepest possible
low-power state. Effectively, software can use this to suspend itself to
RAM.

Unfortunately, there really is no good way to implement a system-wide
PSCI call in KVM. Any precondition checks done in the kernel will need
to be repeated by userspace since there is no good way to protect a
critical section that spans an exit to userspace. SYSTEM_RESET and
SYSTEM_OFF are equally plagued by this issue, although no users have
seemingly cared for the relatively long time these calls have been
supported.

The solution is to just make the whole implementation userspace's
problem. Introduce a new system event, KVM_SYSTEM_EVENT_SUSPEND, that
indicates to userspace a calling vCPU has invoked PSCI SYSTEM_SUSPEND.
Additionally, add a CAP to get buy-in from userspace for this new exit
type.

Only advertise the SYSTEM_SUSPEND PSCI call if userspace has opted in.
If a vCPU calls SYSTEM_SUSPEND, punt straight to userspace. Provide
explicit documentation of userspace's responsibilites for the exit and
point to the PSCI specification to describe the actual PSCI call.

Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220504032446.4133305-8-oupton@google.com

1 files changed, 39 insertions, 0 deletions

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 46ca84600dcab..d8d7859fc556e 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6015,6 +6015,7 @@ should put the acknowledged interrupt vector into the 'epr' field.
   #define KVM_SYSTEM_EVENT_RESET          2
   #define KVM_SYSTEM_EVENT_CRASH          3
   #define KVM_SYSTEM_EVENT_WAKEUP         4
+  #define KVM_SYSTEM_EVENT_SUSPEND        5
 			__u32 type;
                         __u32 ndata;
                         __u64 data[16];
@@ -6042,6 +6043,34 @@ Valid values for 'type' are:
  - KVM_SYSTEM_EVENT_WAKEUP -- the exiting vCPU is in a suspended state and
    KVM has recognized a wakeup event. Userspace may honor this event by
    marking the exiting vCPU as runnable, or deny it and call KVM_RUN again.
+ - KVM_SYSTEM_EVENT_SUSPEND -- the guest has requested a suspension of
+   the VM.
+
+For arm/arm64:
+^^^^^^^^^^^^^^
+
+   KVM_SYSTEM_EVENT_SUSPEND exits are enabled with the
+   KVM_CAP_ARM_SYSTEM_SUSPEND VM capability. If a guest invokes the PSCI
+   SYSTEM_SUSPEND function, KVM will exit to userspace with this event
+   type.
+
+   It is the sole responsibility of userspace to implement the PSCI
+   SYSTEM_SUSPEND call according to ARM DEN0022D.b 5.19 "SYSTEM_SUSPEND".
+   KVM does not change the vCPU's state before exiting to userspace, so
+   the call parameters are left in-place in the vCPU registers.
+
+   Userspace is _required_ to take action for such an exit. It must
+   either:
+
+    - Honor the guest request to suspend the VM. Userspace can request
+      in-kernel emulation of suspension by setting the calling vCPU's
+      state to KVM_MP_STATE_SUSPENDED. Userspace must configure the vCPU's
+      state according to the parameters passed to the PSCI function when
+      the calling vCPU is resumed. See ARM DEN0022D.b 5.19.1 "Intended use"
+      for details on the function parameters.
+
+    - Deny the guest request to suspend the VM. See ARM DEN0022D.b 5.19.2
+      "Caller responsibilities" for possible return values.
 
 If KVM_CAP_SYSTEM_EVENT_DATA is present, the 'data' field can contain
 architecture specific information for the system-level event.  Only
@@ -7767,6 +7796,16 @@ At this time, KVM_PMU_CAP_DISABLE is the only capability.  Setting
 this capability will disable PMU virtualization for that VM.  Usermode
 should adjust CPUID leaf 0xA to reflect that the PMU is disabled.
 
+8.36 KVM_CAP_ARM_SYSTEM_SUSPEND
+-------------------------------
+
+:Capability: KVM_CAP_ARM_SYSTEM_SUSPEND
+:Architectures: arm64
+:Type: vm
+
+When enabled, KVM will exit to userspace with KVM_EXIT_SYSTEM_EVENT of
+type KVM_SYSTEM_EVENT_SUSPEND to process the guest suspend request.
+
 9. Known KVM API problems
 =========================